GENEVA (Reuters) - The World Health Organization expressed concern on Friday about the emergence of a new strain of the Ebola virus that has infected 51 people and killed 16 in western Uganda.

The outbreak, announced by U.S. and Ugandan health officials on Thursday, is in Bundibugyo, near the border with Democratic Republic of Congo.

Genetic analysis of samples taken from some of the victims shows it is a previously unknown type of Ebola, making it the fifth strain, they said.

"We do not yet know yet exactly the lethality of this virus because we haven't tested all the samples," WHO spokesman Gregory Hartl told a news briefing in Geneva.

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"But, obviously, anytime we're looking at Ebola and a new strain of Ebola, this presents a new challenge. So we're very concerned."

Ebola is a hemorrhagic fever, meaning it can cause internal and external bleeding. Victims often die of shock but symptoms can be vague, including fever, muscle pain and nausea.

It is known to infect humans, chimpanzees and gorillas.

Ugandan health officials have said the new strain appears to be relatively mild, but Dr Tom Ksiazek of the U.S. Centers for Disease Control and Prevention said it was not yet clear whether this was the case. He said experts need to check to see how many diagnosed patients are still alive.

Patients with the new strain showed some clinical symptoms, including vomiting, that differed from the usual ones, Hartl said.

"We are very concerned about this because it does not present (clinically) in exactly the same way as other Ebola strains," he said.

Health officials are trying to trace all possible cases in order to test them and isolate them if necessary, he added.